Unless otherwise indicated herein, the description provided in this section is not itself prior art to the claims and is not admitted to be prior art by inclusion in this section.
In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality (whether or not operated by a human user). In general, each coverage area may operate on one or more carriers each defining one or more ranges of frequency spectrum and having a respective “downlink channel” for carrying communications from the base station to UEs and a respective “uplink channel” for carrying communications from the UEs to the base station. Such carriers may be frequency division duplex (FDD), in which the downlink and uplink channels are defined as separate respective ranges of frequency, or time division duplex (TDD), in which the downlink and uplink channels are defined on a common range of frequency but distinguished through time division multiplexing. Further, the downlink channel and uplink channel of each carrier may also be divided into respective sub-channels for carrying particular communications, such as one or more control channels for carrying control signaling and one or more traffic channels for carrying application-layer data and other traffic.
For instance, in a system operating according to an orthogonal frequency division multiple access (OFDMA) protocol, such as the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS) for example, the air interface is divided over time into frames and sub-frames each defining two slots, and the uplink and downlink channels are each divided over their frequency bandwidth into sub-carriers that are grouped within each slot into resource blocks. When a UE is positioned within coverage of a base station in such a system, the UE may register or “attach” with the base station on a particular carrier on which the base station is configured to provide, and the base station may then schedule particular downlink and uplink resource blocks on that carrier to carry data communications to and from the UE.
Under certain air interface protocols, a base station may be able to serve a UE concurrently on multiple carriers, to help increase the effective bandwidth and associated throughput available to the UE. This is known as carrier aggregation. By way of example, according to the LTE air interface protocol, the maximum bandwidth for a data transaction between a base station and a UE using a single carrier frequency is 20 MHz. By engaging in carrier aggregation, the base station may increase the number of resource blocks provided to a UE by aggregating up to five carrier frequencies, and consequently increasing the maximum bandwidth to up to 100 MHz. To facilitate carrier aggregation service, the base station may designate one carrier as a primary carrier or primary cell (PCell) and the base station may designate each other carrier as a secondary carrier or secondary cell (SCell). Further, each aggregated carrier may be generally referred to as a component carrier.
While a base station serves a UE on a single carrier or on multiple carriers, the base station and UE may modulate their air interface data communications at a coding rate selected based on the UE's channel quality, such as with higher coding rate when the UE has better channel quality and with a lower coding rate when the UE has worse channel quality. Further, while the base station serves the UE, the UE's channel quality may change for a variety of reasons such as, for example, due to channel fading or multipath. To determine the channel quality for a UE, the base station may employ a channel state feedback reporting procedure. For instance, in an LTE system, the UE periodically reports to the base station channel state information (CSI), which includes, among other things, a channel quality indicator (CQI) that takes into account factors such as signal-to-noise ratio, antenna configuration, UE receiver type, and so forth. The UE determines the CSI based, at least in part, on downlink transmissions and the UE reports the CSI to the base station using uplink channels at regular intervals specified by the base station (i.e., according to a specified “CSI reporting rate”). The base station may then determine from the reported CSI a particular modulation and coding scheme (MCS) to use for communication with the UE, and the base station may allocate air interface resources and schedule transmissions of data accordingly. It is thus beneficial for the base station to have accurate and up-to-date information of the prevailing channel quality for a UE so that the base station can make appropriate decisions as to resource allocation and MCS.